Applicant's U.S. Patent Application Publication Ser. No. 2006/0127102, which was filed Dec. 10, 2004 and allowed Nov. 19, 2009, teaches methods and systems for controlling a transmitter capable of synthesizing an arbitrary optical E-field waveform. FIG. 1a schematically illustrates a transmitter 2 implementing methods in accordance with in accordance with Applicant's U.S. Patent Application Publication Ser. No. 2006/0127102. As may be seen in FIG. 1, a signal synthesizer 4 provides a high-speed signal path comprising a signal processor 6, a linearizer 8, and a pair of Digital-to-Analog Converters (DACs) 10x, where ‘x’ is an index. The Signal processor 6 receives a data signal X(t) as an input, and uses a compensation function C[ ] to compute parallel multi-bit In-phase and Quadrature sample streams (EI(n) and EQ(n), respectively) representing successive loci of the end-point of a desired or target optical E-field vector. The linearizer 8 then uses the multi-bit (EI(n), EQ(n)) loci to synthesize a pair of multi-bit digital drive signals VR(n) and VL(n). The digital drive signals VX(n) are then converted into analog (RF) signals by respective high speed multi-bit Digital-to-Analog Converters (DACs) 10, and then amplified (and possibly band-pass filtered to remove out-of-band noise) to generate a pair of parallel analog drive signals SX(t), which are output from the synthesizer 4.
The analog drive signals SX(t) output from the synthesizer 4 are supplied to a complex Electrical-to Optical (E/O) converter 12 to generate an optical E-field EO(t) at the complex E/O converter output 14. An optical coupler 16 and detector 18 samples the output optical E-field EO(t), and supplies the samples to a controller unit 20, which detects an error between the actual output optical E-field EO(t) and the desired complex E-field waveform as represented by the parallel multi-bit In-phase and Quadrature sample streams (EI(n) and EQ(n). The controller unit 20 then adjusts at least one parameter of the transmitter to minimize the detected error. With this arrangement, controller 20 operates to control the transmitter to produce an optical E-field EO(t) at the complex E/O converter output 14 which is a high-fidelity reproduction of the target E-field computed by the signal processor 6.
In general, the signal processor 6 is capable of implementing any desired mathematical function, which means that the compensation function C[ ] can be selected to compensate any desired signal impairments, including, but not limited to, dispersion, Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), four-wave mixing and polarization dependent effects (PDEs) such as polarization dependent loss. In addition, the compensation function C[ ] can be dynamically adjusted for changes in the optical properties of the link, and component drift due to aging. The inherent flexibility of the mathematical function implemented by the signal processor 6 also implies that the signal processor 6 can be placed into a “test” mode, and used to generate (EI(n), EQ(n)) loci of a desired optical E-field vector independently (or even in the absence) of an input data signal X(t).
The linearizer 8 can also implement any desired mathematical function, and thus can perform signal format conversion (i.e. from Cartesian to polar coordinates); compensate for non-linearities in the signal path between the linearizer 4 and the output 14 of the complex E/O converter 12; and perform various scaling and clipping operations to limit dynamic range requirements of electrical components downstream of the linearizer 8 (principally the DACs 8).
The resolution of each analog drive signal SX(t) is governed by that of the DACs 8. In general, each DAC 8 has a resolution of M-bits, where M is an integer, which yields excursions of each analog drive signal SX(t) between 2M discrete levels. It will be noted that M=1 represents a trivial case, in which each analog drive signal SX(t) is a bi-state signal. In preferred embodiments, M is greater than 4.
The complex E/O converter 12 will normally be provided as either a conventional dual branch MZ interferometer, or as nested MZ interferometers, both of which are known in the art. In either case, a laser 22 is driven to produce a narrow-band optical carrier, which is supplied to each branch 24 of the E/O converter 12. The resulting modulated branch signals are then combined at the output 14 of the E/O converter 12 to produce the optical E-field EO(t). FIG. 1b illustrates an alternative arrangement, in which the modulated branch signals in each branch 24 of the E/O converter 12 are combined using a 2×2 signal combiner 26. As is known in the art, such a combiner generates two output signals, both of which are the product of mixing to the modulated branch signals from each branch 24. One of these output signals may be designated as a “main” signal, and is coupled to the output 14 of the E/O converter 12 as the output optical signal EO(t). The other signal may be designated as a “complementary” signal, and is supplied to the detector 18. This arrangement is beneficial in that the complementary signal enables error detection and control of the transmitter, without requiring a separate splitter 16 to sample the output signal EO(t).
Multi-bit digital generation of the drive signals SX(t) in this manner enables the optical transmitter to synthesize any desired E-field waveform at the output 14 of the complex E/O converter 12. Because the linearizer 8 synthesizes the digital drive signals VX(n) based on a model of the target optical E-field (as opposed to the data signal being transmitted), it is possible to derive a mathematical representation of the entire data path between the signal processor 6 and the E/O converter output 14, which enables phase and amplitude of the output E-field EO(t) to be independently controlled, even with significant coupling of phase and amplitude responses of the complex E/O converter 12.
The disclosure of U.S. Patent Application Publication Ser. No. 2006/0127102 focuses on embodiments in which the electrical-to-optical (E/O) converter 12 generates the output optical E-field as either an un-polarized or a linearly polarized signal having a single polarization. As is known in the art, one method of increasing the line rate of an optical channel is by generating two independent linearly polarized optical signals having the same carrier wavelength and orthogonal polarization angles. The two polarized optical signals can then be polarization-multiplexed together to produce a polarization multiplexed optical signal for transmission through an optical fibre link to a receiver. Since the two orthogonal polarization signals are independent, the techniques described in U.S. Patent Application Publication Ser. No. 2006/0127102 may be applied to the case of a dual polarization transmitter, by duplicating the system of FIG. 1 for each polarization. FIG. 2 schematically illustrates a dual polarization transmitter 28 of this type.
Referring to FIG. 2, the dual polarization transmitter 28 comprises, for each of the X- and Y-polarizations, a respective transmitter 2 as described above with reference to FIG. 1. Thus, the X-Polarization transmitter 2X includes a respective synthesizer 4X which generates a pair of analog drive signals SXR, SXL for driving a corresponding complex E/O converter 12X to output a X-Polarization optical signal EX, as described above with reference to FIGS. 1a and 1b. The Y-Polarization transmitter 2Y is configured in a substantially identical manner, and outputs a corresponding Y-Polarization optical signal EY. The polarization optical signals EX and EY are then combined using a polarization beam combiner 30 to yield a polarization multiplexed optical signal EOUT at an output 32 of the transmitter 28. In the embodiment of FIG. 2, a common laser 22 is used to generate a narrow-band carrier light, which is then split into respective carrier lights for each of the X- and Y-Polarizations using a polarization beam splitter 34.
The arrangement of FIG. 2 provides a high degree of control over the parameters of each polarization signal EX and EY. Thus, for example, a dual polarization transmitter 28 implementing the methods of U.S. Patent Application Publication Ser. No. 2006/0127102 is capable of generating a polarization multiplexed optical signal EOUT, in which each of the orthogonal polarization signals EX and EY may have any desired E-field envelope, limited primarily by the dynamic range of each polarization transmitter 2.
It is frequently desirable to be able to measure the optical performance of a dual polarization transmitter. For example, during manufacture of the transmitter, it is necessary to ensure that it is operating properly. The methods of U.S. Patent Application Publication Ser. No. 2006/0127102 are capable of compensating non-ideal performance of the transmitter, and so can accommodate performance variations within normal manufacturing tolerances. However, it is still necessary to characterise the optical performance of each transmitter, and ensure that it falls within the design specifications.
Typically, the optical performance of a dual polarization transmitter is measured by tapping optical signals at various points in the transmitter, and supplying the tapped optical signals to a set of specialized optical analysis equipment. For example, optical spectrum analysers can be used to measure the spectral response of each polarization. Other types of test and measurement equipment, such as oscilloscopes, phase detectors etc. may be used to measure other optical performance parameters of the transmitter.
However, optical signal analysis equipment of this type tends to be very expensive. This equipment also tends to be quite bulky, and can be fairly fragile. In a manufacturing environment, these factors tend to increase the manufacturing costs of the transmitters. The bulk and sensitivity of the equipment severally limits its mobility, and so makes it very difficult to analyse the performance of transmitters in the field, for example during System Layout and Test (SLAT), or maintenance of installed transmitters.
Techniques that enable cost effective evaluation of dual polarization transmitters remain highly desirable.